1. Field of the Invention
The present invention relates to a capacitor construction for use in pressure transducers.
2. Related Prior Art
References to the prior art include the following patent publications:
[1] U.S. Pat. No. 4,589,054, (Kuisma); PA1 [2] U.S. Pat. No. 4,597,027, (Lehto); PA1 [3] U.S. Pat. No. 3,397,728, (Pomerantz); PA1 [4] U.S. Pat. No. 4,609,966, (Kuisma); PA1 [5] U.S. Pat. No. 4,599,906, (Freud et al.); PA1 [6] U.S. Pat. No. 4,542,435, (Freud et al.); PA1 [7] U.S. Pat. No. 4,257,274, (Shimade et al.); PA1 [8] U.S. Pat. No. 4,628,403, (Kuisma); PA1 (a) precision temperature compensation by design of the compensation element into the sensor construction, PA1 (b) improved adaptability to mass production and better bondability than with differential capacitors fabricated from similar materials, (see references ([63, [73), and PA1 (c) as known from the reference [8], the advantageous dependence of pressure sensitivity in a capacitor is such that it results in a higher sensitivity at low pressures compared with the sensitivity at higher pressures. This property can also be utilized in conjunction with a differential pressure sensor, whereby the pressure P.sub.1 in channel 22 must be greater than the pressure P.sub.2 in channel 23 (FIG. 3). Compared to symmetrical constructions of differential capacitors shown in references [6] and [7], we now attain a wider usable measurement span with a single sensor element.
A disadvantage of constructions of prior art technology is that they are difficult to implement in mass production. Further, elimination of temperature-dependent errors to a reasonable level has previously been unsuccessful.
The aim of the present invention is to overcome the disadvantages of the prior art technology and achieve a totally new kind of capacitor construction for use in pressure transducers.
The invention is based on use of a second capacitor integrated to the capacitor construction for temperature compensation of the transducer. A particularly advantageous placement of the compensation capacitor's fixed plate is on the same substrate next to the fixed plate of the sensor capacitor.
More specifically, the capacitor construction in accordance with the invention is characterized by what is stated in the characterizing part of claim 1.
The invention provides outstanding benefits, including:
It must be noted that the construction in accordance with the invention not only offers compensation of temperature dependence on the dielectric properties of the insulating material but also cancels other causes of temperature-dependent errors, such as those caused by elastic stress properties and thermal expansion characteristics, as well as the thermal expansion of the silicon oil pressure medium if the isolation diaphragms are located close to the sensor element and in good thermal contact with it.
The capacitor construction in accordance with the invention achieves appreciable benefits when the sensor element is attached between the two metal shells of the sensor case using an elastomer so that the sensor capacitor floats between the elastomer cushions. By virtue of its high elasticity, the elastomer does not transmit to the sensor capacitor the mechanical stress caused by the deformation of the metal case or by the differential thermal expansion related to the different expansion coefficients of the sensor capacitor material and the case metal. The location of the sensor capacitor between the two supporting surfaces is fixed so that the imposed pressure difference to be measured cannot stretch the elastomer layer excessively in the direction of its thickness. The shear strength of the elastomer is increased to a sufficient level by using a thin layer in conjunction with a large area.
The invention is next examined in detail with help of the following exemplifying embodiment illustrated in the attached figures.